In response to the recent trends of increasing the degree of integration and speed of LSI, the design rule of an LSI semiconductor device are being further reduced. Such reduced design rule is also applied to, e.g., planar type transistors. However, as the dimension of a planar type transistor becomes smaller, problems involving increased leakage current, higher power consumption and higher driving voltage arise. Therefore, it is thought that a limit exists in increasing the degree of integration beyond a 32-nanometer node. In this regard, transistors having a three-dimensional (3D) structure such as a fin structure, a recess gate structure and a double gate structure are recently being developed.
On the other hand, in the fabrication of such a conventional planar type transistor, a silicon oxide film (SiO2 film) used as a gate insulating film is nitrided such that nitrogen is introduced into the film, thus forming a silicon oxynitride film (SiON film) (see, e.g., Japanese Patent Application Publication Nos. H9-148543 and H10-32328). It is known that, by forming the silicon oxynitride film when forming the silicon oxynitride film, the dielectric constant is increased to thereby reduce an equivalent oxide thickness (EOT) and leakage current.
Further, also in the above-mentioned 3D-structured transistors, EOT and leakage current are expected to decrease as in the conventional planar type transistor by using the silicon oxynitride film as the gate insulating film in place of the silicon oxide film. However, in case of the 3D-structured transistor, the gate insulating film also is formed in a three dimensional structure. Therefore, the concentration of nitrogen introduced by nitridation may differ between a sidewall portion and a planar portion in the gate insulating film. Since the nitrogen concentration in the gate insulating film affects the electrical properties of the device, it is necessary that the nitrogen concentration of each of the sidewall and the planar portion in the 3D-structured gate insulating film be accurately measured. Further, in nitriding process, the process conditions need to be determined such that each portion in the resulting silicon oxynitride film shows its desired level of nitrogen concentration.
For measuring the nitrogen concentration in the silicon oxynitride film, XPS (X-ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry), for example, are being used. Although these methods are suitable for measuring a planar silicon oxynitride film, it is difficult to measure the nitrogen concentration only in the sidewall portion with a high accuracy by applying these methods to a silicon oxynitride film of a three dimensional shape in a 3D-structured device. This is because, when performing a measurement of the nitrogen concentration in the sidewall portion by the above methods, the sum of the nitrogen concentrations in the sidewall portion and in the planar portion is obtained as the measurement result instead of the nitrogen concentration only in the sidewall portion.